Medium pumps for displacing liquid media have been known for a long time, in particular in form of lubricating oil pumps for motor chain saws or the like (see for example DE-A1-41 07 110 or the DE-A137 21 643 or the DE-A1-34 47 091 or the DE-Cl-1 95 29 368). Such lubricating oil pumps in chain saws are complex structural components which are assembled by the manufacturer and which can be maintained by the user only by means of “special tools”. Since the displaced oils tend to gum, it is necessary from time to time to disassemble the pump and to make it free from the gum accumulations. For the pumps known at present, this is only possible with the aid of special tools. Since the user of the chain saw does not have such special tools or since he does not have them continuously on him, this can mean a nonworking time.